file: use _get_tempfile() in metric storage
file: use _get_tempfile() in metric storage

This changes the temporary file used to be stored in the specific
temporary directory we now use.

Change-Id: I3899c1cbe168241d2b394033e470cb9428ba33fb

sileht/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi,leandroreox/gnocchi